Lec 8 - Basal ganglia

• Basal ganglia (aka basal nuclei) are a subcortical group of white matter structures deep to the temporal lobes
• The function of the basal ganglia is to receive cortical input, provides negative feedback to cortex to modulate (fine tune) motor movement
• The caudate nucleus and the putamen together make up the dorsal striatum and the nucleus accumbens makes up the ventral striatum. Both dorsal and ventral striatum's can collectively be referred to as the striatum
• Label the following basal ganglia and related structures

  A) caudate nucleus

  B) globus pallidus

  C) putamen

  D) dorsal striatum

  E) thalamus

  F) subthalamic nucleus

  G) amygdala

  H) substantia nigra

  I) nucleus accumbens

  J) ventral striatum

  
• Basal ganglia are a group of interconnected nuclei that receive information mainly from premotor areas and send processed information back to premotor areas via thalamus
• Basal ganglia main function is to regulate motor and premotor cortical areas, facilitating smooth voluntary movements. Also involved in memory and cognition patterns of movement
• Premotor areas send signals to basal ganglia which come up with a pattern of movements that will enable that action. That pattern is sent to the thalamus (relay station) then back to the premotor cortex. From there, the signal travels to the primary motor cortex to spinal cord and brainstem and then the motor function/action is executed
• The caudate nucleus has a head, body, and tail. The tails ends at the amygdala.

  A) head

  B) body

  C) tail

  D) amygdala

  E) putamen

  F) thalamus

  
• The caudate nucleus' role is integration of spatial positioning to fine tune motor response. Also partly contributes to speed of response
• The globus pallidus consists of an internal and external segment. The internal division has a motor function and the external division has a sensory function. It plays an essential role in the modulation of motor input
• The subthalamic nucleus is a small paired structure below the thalamus. It is considered the pacemaker of the basal ganglia
• The substantia nigra is a small motor nucleus in the anterior part of the midbrain. It is important in dopamine response within the basal ganglia
• The putamen and the globus pallidus together are called the lentiform nucleus
• The internal capsule is white matter tracts that are lateral to thalamus and caudate nucleus and media to the lentiform nucleus. It is the connection of the cerebral cortex to subcortical structures and the brainstem and spinal cord.
• The subthalamic nucleus is considered the pacemaker of the basal ganglia because of rate-limiting enzymes. It decides how fast the process goes
• Basal ganglia input areas

  • dorsal striatum - caudate nucleus & putamen
  • ventral striatum - nucleus accumbens
  • subthalamic nucleus
• Basal ganglia output areas

  • globus pallidus
  • substantia nigra
• The basal nuclei modulate motor function through various pathways to initiate, terminate, or adjust the extent of the movement. There are two major pathways that connect input areas to output areas

  • direct
  • indirect
• Cortical input and output is regulated by the thalamus
• The thalamus is under chronic inhibition
• Release-inhibition model - act in parallel not one or the other

  • direct: releases the inhibition of the thalamus to result in more cortical output which leads to more motor output
  • indirect: enhances the inhibition of the thalamus to result in less cortical output which leads to less motor output
• Direct pathway is an excitatory pathway that facilitates target-oriented movements
• Direct pathway

  • cerebral cortex releases excitatory glutamate and substantia nigra via the nigrostriatal dopaminergic pathway has an excitatory effect via D1 receptors on the striatum
  • Striatum releases GABA (inhibitory) onto internal globus pallidus
  • internal globus pallidus releases GABA which disinhibits the thalamus
  • thalamus is free to send excitatory signals to the motor cortex
  • results in cortex excitation and more motor output
• Indirect pathway is inhibitory and inhibits competing movements
• Indirect pathway

  • cerebral cortex sends excitatory NT glutamate and substantia nigra via nigrostriatal dopaminergic pathway has an inhibitory effect via D2 receptors on striatum
  • striatum releases GABA and inhibits external globus pallidus
  • this disinhibits the subthalamic nucleus which is now free to send excitatory NT to internal globus pallidus
  • this further inhibits the thalamus and cannot send excitatory signals to the motor cortex resulting in less motor output
• Outputs of direct and indirect pathways from basal ganglia to motor cortex are opposed = balanced. Disturbance of output upsets the balance of motor control, leads to increase or decrease of movement. Ex. huntington's or parkinson's disease
• Parkinson's disease is abnormal involuntary movements, decreased movements from cellular damage of the substantia nigra therefore lacking dopamine. Symptoms include

  • rigidity
  • postural instability
  • bradykinesia
  • tremors
• Dopamine stimulates the direct pathway and inhibits the indirect pathway. Activation of the thalamus (via both direct and indirect) leads to activation of the motor cortex. Dopamine also helps basal ganglia neurons to learn patterns of motor programs. Lack of dopamine will result in reduced inhibition via indirect pathway and reduced excitation via direct pathway
• Hyperkinesia movement disorder = Huntington's disease

  This occurs from the degeneration of striatal GABAergic neurons. Presents with chorea (involuntary jerky movements), dementia